The present invention relates generally to the rolling of hot metal workpieces and more particularly to the determination of the workpiece temperature as it enters the first stand of a finishing mill.
In order to roll quality on-gage metal sheets and bars, a number of properties of the metal (e.g., steel) to be rolled should be accurately known. Primary among these properties, in the case of a hot rolling mill, are the material's composition, its thickness, its width and its temperature as it enters the finishing mill. The composition of the material is normally known before any rolling operation begins. Width and thickness are physical parameters which are fairly easily determined by various methods including physical measurements at any of several places in the mill. Temperature presents a more difficult problem in that pyrometers generally in use today are capable of providing accurate measurements only when the bar's surface is relatively clean and devoid of the scale which forms when the heated material is exposed to air. Other methods of temperature determination, in today's technology, are generally no more accurate than well tuned pyrometers in determining bar temperature.
The most common method of rolling hot metal strip involves heating an ingot or slab to a fairly high temperature (e.g., 1300.degree. C) and then reducing the thickness of the bar by subsequent passing of the slab through a series of rolling mill stands. The rolling sequence normally takes place in two stages commonly referred to as the roughing mill and the finishing mill. The roughing mill may be either of the tandem type or a reversing mill employing a single stand. In either case the slab or ingot is subjected to repeated operations in the mill until its thickness is reduced to some prescribed value, normally in the range of 1 to 10 centimeters, at which time it leaves the roughing mill and is transferred by way of a delay table to the finishing mill. The finishing mill is usually of the tandem type wherein the bar, as it is now customarily called, is further reduced in thickness to sheet of the desired thickness or gage. It is in the finishing mill where the previously mentioned parameters of composition, width, thickness and temperature become very important to the accurate rolling of on-gage material in order to "set up" the mill to achieve the proper rolling. As an example of a method by which a finishing mill is set up, reference is made to U.S. Pat. No. 26,996, "Computer Control System For Metals Rolling Mill" by R. G. Beadle et al (issued Dec. 8, 1970) which patent is assigned to the assignee of the present invention.
As previously mentioned, pyrometers in general use today normally need a clean surface in order to accurately determine temperature. Because a hot bar develops surface scale fairly rapidly when exposed to air, it is proven impractical to provide a direct measurement of the bar temperature as it enters the finishing mill. Standard practice, therefore, in the past has been to measure the temperature of the head end of the bar as it exits the roughing mill where it is relatively free of scale. The length of time the bar remains on the delay table is then measured and using well-known heat transfer equation, the temperature of the bar as it enters the finishing mill is determined. This method has proven quite satisfactory for standard mills.
There are, however, a number of other problems associated with such standard mills, particularly in association with the delay table. For example, such a delay table must be longer than the actual length of the bar as it emerges from the roughing mill. This, of course, affects the size of the physical plant required. In addition, because the bar exit speed is must higher than the finishing mill entry speed (e.g., 4 times as high), the tail end of the bar will normally remain on the table for a longer period of time with a resultant greater loss of heat. As such, the tail of the bar enters the finishing mill at a lower temperature than the head thus requiring compensation during rolling within the finishing mill.
The length of time that a bar spends on the delay table is also related to the amount of scale which is developed on the bar which in turn determines the amount of descaling required before the bar enters into the finishing mill. As a method of descaling, high pressure water sprays are often used and it is readily seen that the greater amount of scaling which occurs, the greater amount of descaling and hence a greater amount of heat loss due thereto.
In an effort to minimize the above problems, a system utilizing a coiler as a part of the delay table has been devised. This system is described in detail in U.S. Pat. Nos. 3,803,891. "Method Of Rolling Hot Metal Workpieces" by W. Smith (issued Apr. 16, 1974) and 3,805,570, "Method And Apparatus For Rolling Hot Metal Workpieces And Coilers For Use In Coiling Hot Metal Workpieces" by W. Smith (issued Apr. 23, 1974), which patents are specifically incorporated hereinto by reference.
The rolling method involving the use of a coiler, while alleviating some of the aforementioned problems, creates others. In this system, the head end from the roughing mill is first into the coiler but the tail end is the first to emerge therefrom. Thus, the tail end from the roughing mill becomes the head end to the finishing mill. Due to the fact that there still exists a difference in temperature between head and tail ends, the temperature measurements and calculations made in the old manner are unsatisfactory. In addition, the heat dissipation from the bar in coil from while in the coiler is considerably different from that experienced when the bar is flat on the open portion of the delay table. Thus, it is seen that the existing method of calculating the finishing mill entry temperature is not satisfactory for this type of mill.